An ankle-foot orthosis (AFO) is commonly used to help subjects with weakness of ankle dorsiflexor muscles due to peripheral or central nervous system disorders. Both of these disorders are due to the weakness of the tibialis anterior muscle which results in lack of dorsiflexion assist moment. The deformity and muscle weakness of one joint in the lower extremity influences the stability of the adjacent joints, thereby requiring compensatory adaptations.
During level ambulation, the ankle should be close to a neutral position (a right angle) each time the foot strikes the floor. Insufficient dorsiflexion may be the result of hyperactive plantarflexion muscles that produce a very high plantarflexion moment at the ankle, or weakness of the dorsiflexion muscles. This affects the ability of the ankle to dorsiflex. Both of these cause the patient to make a forefoot contact instead of the normal “heel-strike”. If there is a weak push-off, the stride length reduces, and the gait velocity fails. Similarly, during the swing phase of the gait, the ankle is dorsiflexed to allow the foot to clear the ground while the extremity is advanced. Hyperactive or weak dorsiflexors may result in insufficient dorsiflexion, which must be compensated for by alterations in the gait patterns so that the toes do not drag. This insufficient dorsiflexion during the swing phase of the gait is termed as “foot-drop”. In addition to the toes dragging, the foot may become abnormally supinated, which may result in an ankle sprain or fracture, when the weight is applied to the limb. Foot-drop is commonly seen in subjects who have had a stroke or who have sustained a personal nerve injury.
There are several possible treatments for foot-drop including medicinal, orthotic, or surgical interventions. Of these, orthotic treatment is the most common. Orthotic devices are intended to support the ankle, correct deformities, and prevent further occurrences. A key goal of orthotic treatment is to assist the patient in achieving a measure of normal function.
There are a number of commercially available ankle-foot orthoses. All, however, are single axis or elastically deformable. While inversion-eversion motion in all of these orthoses is accommodated through the flexibility of the material, such as polypropylene, the limitation in normal inversion-eversion does not provide a natural motion to the ankle and adds discomfort. Thus, there is a need for an ankle foot orthosis allowing for a more natural motion of the ankle during movement.